The long range objective of this project has been to characterize, in a biochemical and genetic sense, virulence associated traits of selected oral microorganisms. Currently, two projects are under study: (A) characterization of plasmids from Streptococcus mutans, and (B) molecular cloning of fimbrial components from Actinomyces viscosus. With respect to project A, we noted that strain V679, which is S. sanguis Challis transformed with the S. mutans plasmid pVA380, had about ten-fold greater glucosyl-transferase (GTF) activity than untransformed Challis. Subsequent research indicated that this increased activity was not due to pVA380 per se, but to a genetic instability of V679. This strain gave rise to two, hemolytically distinct, types of colonies. Type 1 colonies had high GTF activity and long chains in broth culture; type 2 colonies had low GTF activity and short chains. Both types harbored pVA380, which indicates that this plasmid was not responsible for the high GTF activity of V679. Type 2 variants were readily obtained upon subculture of type 1 colonies. It is likely that the simultaneous change in hemolysis, GTF, and chain length was due to a pleiotropic mutation affecting the cytoplasmic membrane or the cell wall. The frequency of this change appeared to be significantly greater than that of classical mutations. In addition, this change was reversible in a phenotypic sense. With respect to project B, plasmid cloning was used to establish a gene library of strain T14V in E. coli. About 550 clones were obtained and analysis showed that virtually each one of them contained a distinct 45 kilo-base fragment of Actinomyces DNA. On a statistical basis (P greater than 0.99), this collection represents the entire genome of strain T14V. The gene library was then screened for the expression of Actinomyces surface antigens. Initially, a few weakly positive clones were detected; these were then used to optomize the detection method. Ultimately, about 24 strongly reacting clones were identified after screening of the whole library. These clones were tested with monospecific anti-sera against type 1 and 2 fimbriae. Clone 1402 reacted strongly with antiserum againset type 2 fimbriae. This clone also reacted with a pool of monoclonal antibodies directed at type 2 fibriae.